In order to decompose pollutants through combustion in a device known as a combustion exchanger (see U.S. Pat. No. 4,741,690), a structure having one upper and one lower air distributing duct is often used. The polluted air passes through the layer of bed material positioned between said ducts, and the bed material often consists of sand which has been pre-heated to an elevated temperature of about 1000.degree. C.
The capacity expressed as quantity of air flow per time unit is determined by the parameters pressure drop and temperature of the bed. The pressure drop is a function of the thickness of the bed, the composition of the material of the bed, the surface structure, the granular size and the compaction degree of the material of the bed, and so on. To obtain a satisfactory degree of purification, expressed as the proportion of pollutants remaining in the exhaust in relation to the amount of pollutants in the incoming gas, a certain dwelling time in the hot zone is required. Each bed layer therefore provides a specific degree of purification for a given flow capacity, depending on the composition and thickness of the bed material. When the velocity of the air passing through the bed is high, the pressure drop becomes considerable. The area of the bed in a combustion exchanger therefore determines the dimensions of the total flow.
Because of the restrictions laid down by the road traffic rules and regulations concerning transports, units manufactured in one place for installation elsewhere, must not exceed certain limits as to their area and as a result they have a restricted flow capacity.
If the combustion exchanging technology is to be used to treat flows larger than those for which the largest units allowable on public roads are intended, larger units, built in situ, may be used as an alternative to prefabricated units. This alternative provides economical advantages as the costs per flow unit become smaller.
Large flows require large bed areas. In one combustion exchanger of conventional construction the polluted air is distributed across a horizontal bed area through an air gap above and below the bed area.
In large-size plants, this construction embodying an air gap would necessitate large spans in the structure forming the roof of the sealed air gap.
Large spans in roofs that are exposed to positive or negative pressures necessitate complicated and thus expensive structures. In addition, the air gap underneath the bed also is a complication in large installations from a maintenance point of view.